CVE-2016-9311 in ntpd
Summary
by MITRE
ntpd in NTP before 4.2.8p9, when the trap service is enabled, allows remote attackers to cause a denial of service (NULL pointer dereference and crash) via a crafted packet.
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Analysis
by VulDB Data Team • 05/13/2026
The vulnerability identified as CVE-2016-9311 affects the Network Time Protocol daemon (ntpd) in versions prior to 4.2.8p9, representing a critical security flaw that can be exploited to execute remote denial of service attacks. This vulnerability specifically manifests when the trap service functionality is enabled within the NTP daemon, creating a pathway for malicious actors to disrupt network time synchronization services. The flaw stems from improper handling of crafted network packets that can trigger a NULL pointer dereference condition, ultimately leading to the complete crash of the ntpd process. This represents a fundamental failure in input validation and error handling mechanisms within the NTP software stack, where the system fails to properly validate incoming packet structures before processing them through the trap service component.
The technical implementation of this vulnerability involves the trap service functionality which is designed to provide network administrators with notifications about specific events occurring within the NTP daemon. When enabled, this service listens for particular network events and responds accordingly, but the implementation lacks proper boundary checks and validation mechanisms. Attackers can craft specially formatted packets that, when processed by the vulnerable ntpd daemon, cause the system to attempt to dereference a NULL pointer, resulting in an immediate crash of the process. This type of vulnerability falls under the Common Weakness Enumeration category CWE-476 which specifically addresses NULL pointer dereference conditions, and represents a classic example of insufficient input validation that can be exploited remotely without requiring authentication or privileged access. The attack vector operates over the network, leveraging the trap service's packet processing capabilities to deliver malicious payloads that trigger the underlying memory management error.
The operational impact of CVE-2016-9311 extends beyond simple service disruption, as it can severely compromise network time synchronization infrastructure that many critical systems depend upon. Organizations relying on accurate timekeeping for security logging, authentication mechanisms, certificate validation, and distributed system coordination face significant risk when this vulnerability exists in their network infrastructure. The denial of service condition can persist until the ntpd process is manually restarted or the system is rebooted, potentially causing cascading failures throughout the network as dependent services lose access to synchronized time references. This vulnerability particularly affects environments where NTP services are exposed to untrusted networks or where the trap service is enabled for monitoring purposes, as these configurations provide the attack surface necessary for exploitation. The lack of authentication requirements for exploitation means that even systems with firewalled NTP services could be vulnerable if the trap service remains enabled, creating a significant risk for organizations with complex network topologies or those operating in hostile network environments.
Mitigation strategies for CVE-2016-9311 should prioritize immediate software updates to versions 4.2.8p9 or later, which contain the necessary patches to address the NULL pointer dereference condition. Network administrators should also consider disabling the trap service functionality entirely if it is not required for operational purposes, as this removes the attack surface associated with the vulnerable code path. Additionally, implementing network segmentation and access controls can help limit exposure by restricting which systems can communicate with NTP services, particularly those with trap service enabled. Security monitoring should include detection of unusual network traffic patterns that might indicate exploitation attempts, and system administrators should establish automated alerting for ntpd process crashes or restarts. The vulnerability also highlights the importance of following security best practices such as implementing principle of least privilege for service configurations, regularly auditing network service configurations, and maintaining up-to-date vulnerability management processes to prevent similar issues from occurring in other network services. Organizations should also consider implementing intrusion detection systems that can identify and alert on suspicious packet patterns consistent with this type of exploit, as the attack does not require authentication and can be executed by any remote attacker with network access to the affected system.